Using Java compiler as preprocessor

February 1, 2007 · Programming

Two days ago, a friend of mine was talking to me about his recent experiences with Java. He insisted that Java needs a preprocessor, because otherwise you are forced to pollute your code with many debug messages, which are very handy when developing, but not as much when you release your application.

I asked him to take a look to some Java preprocessor implementation that are available freely on the internet, and also pointed him to use log4j or another logging facility.

I also had a second thought, a way to use the compiler's optimization process to benefit you with these kind of problems. Consider the following code:

public class Test {
	private static final boolean DEBUG = false;

	public static void main(String[] args) {
		if(DEBUG) {
			System.out.println("debug is on");
		}
	}
}

We set the DEBUG class attribute as false or true, when we want to enable ... disable the debug information. The disassembled code with debug (DEBUG = true) is listed below:

nefarian:~ bkarak$ javap -c Test
public class Test extends java.lang.Object{
public Test();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."":()V
   4:   return

public static void main(java.lang.String[]);
  Code:
   0:   getstatic       #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:   ldc     #3; //String debug is on
   5:   invokevirtual   #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   8:   return

}

and without (DEBUG = false):

nefarian:~ bkarak$ javap -c Test       
public class Test extends java.lang.Object{
public Test();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."":()V
   4:   return

public static void main(java.lang.String[]);
  Code:
   0:   return

}

The Java compiler deletes this code because it is considered unreachable statement. Nice isn't it? :)